Summer Wars (2009)

High school student Kenji Koiso is an honest and introverted math genius. One summer day, he was invited to work in the hometown of his beautiful senior student, Natsuki Jinnai. It turns out that the part-time job involves pretending to be Natsuki's boyfriend. The ancestors of the Jinnai family were once a samurai family that fought fierce battles with the Tokugawa legions. Hundreds of years have passed, and they have become tough and tenacious...